In vivo synthesis, export and degradation of proteins have been studied in the livers of normal adult mice, and during the growth observed: (1) after a partial hepatectomy, (2) when protein-depleted mice are fed an adequate diet, and (3) during the normal developmental growth of newborn mice. The results provide conclusive evidence of the predominant role of protein degradation in the regulation of liver mass. We are now focusing our attention on the mechanisms and control of protein degradation. In continuing these studies our immediate aims are: (1) To study the effect of liver growth on the in vivo degradation of individual proteins, or classes of proteins, including: (a) selected short-lived and long lived liver proteins; (b) abnormal proteins; (c) proteins taken up by endocytosis. (2) To attempt to reproduce the differences in protein metabolism we have observed during liver growth in vivo, in simpler systems, including (a) liver slices; (b) isolated liver cells, freshly prepared or in primary cultures.